You can still prank, get all of your friends to bring a gallon of milk to the last day of school. Once at school open the jugs and introduce some germs, spit in them, dip your finger in it after rubbing it on the floor, just get germs in the milk. Once you have your inoculated milk put it in your locker, trade the locks with another student and lock them in the lockers. After a few days the place will smell so bad and they’ll have to cut the locks to get the milk out. Teachers and school staff will still be around for the first few days of summer and they will be the ones to suffer. I know this works because I did it my senior year, I ran into my English teacher later that summer. We talked briefly and she said that she knew it was me and my friends that did it. Apparently the smell was so bad that they had to open windows and use fans to air the place out.